Reactive oxygen species (ROS) are natural byproducts of oxidative respiration and can function in signal transduction and clearance of pathogens during innate immune responses. Cancer cells have been reported to harbor higher levels of ROS than non-transformed cells, and in some cases activation of a specific oncogene (for example, HRAS) is sufficient to elevate levels of ROS (1, 2). Since ROS are capable of damaging crucial cellular macromolecules, including DNA, some cancer cells may be faced with chronic ‘oxidative stress’ that requires active enzymatic ROS detoxification to prevent induction of cell death. As such, one consequence of some of the genomic alterations leading to tumorigenesis may be a dependency on pathways facilitating the detoxification of ROS for survival, a form of ‘nononcogene addiction’ or ‘non-oncogene co-dependency’ (3-5). Importantly, this dependency might not be shared by many non-transformed cells, whose lower basal ROS levels and/or elevated antioxidant capacity could provide resistance to treatments that impair ROS metabolism. Various small molecules, including many with disulfide, -unsaturated carbonyl, sulfonate, or other electrophilic functional groups, have previously been shown to elevate ROS levels and induce cancer cell death (6). A subset of such compounds has also demonstrated a degree of selective toxicity toward cancer cells in in vitro and in vivo models (7-12).
A cell-based, high-throughput screening approach was used to identify piperlongumine (PL), a naturally occurring, electrophilic small molecule capable of selectively killing a variety of transformed cell types while sparing primary normal cells (5). Piperlongumine's in vivo antitumor efficacy was illustrated in mouse models of cancer, including xenograft and spontaneous mammary tumor formation models. Mechanistic investigations correlated the observed selective toxicity with a cancer-selective increase in ROS and other markers of oxidative stress following treatment with PL, as well as increases in DNA damage and apoptotic cell death (FIG. 1A). The small-molecule nucleophile and antioxidant N-acetyl-L-cysteine prevents PL-mediated cell death, and several proteins known to bind glutathione and detoxify ROS were identified as potential cellular interaction partners of PL through affinity purification and quantitative proteomics.
Piperlongumine analogs where methoxy groups of piperlongumine are substituted with hydrogen, hydroxy, methyl, or other groups have been described (UA 2009/0312373; UA 2011/0053938; UA 2012/0059004, UA 2012/0157455; Duh et al. J. Nat. Prod. 1990 November-December, 53(6) 1575-1577; Duh et al., Phytochemistry 1990, 29: 2689-2691; Raj et al. Nature 475, 231-234 (2011)).